The present invention relates to a system for adjusting the span of an electronic weighing apparatus or scale, and more particularly to an electronic scale span adjusting system capable of easily and accurately correcting the span of an electronic scale according to a variation in an output voltage from a weight sensor.
FIG. 1 of the accompanying drawings is a span adjustment circuit in a conventional electronic scale.
Designated in FIG. 1 at A is a weight sensor comprising a load cell, B is an amplifier for amplifying an output voltage from the weight sensor A, R1 and R2 are voltage-dividing resistors for dividing an output from the amplifier B, C is an A/D converter of the double-integrating-type, and Vr is a variable resistor having input terminals connected across the voltage-dividing resistor R1 and an output terminal to an input terminal connected of the A/D converter C.
The variable resistor Vr has a resistance selected to be much larger than that of the voltage-dividing resistor R1 so that any variation in the resistance of the variable resistor Vr in the combined resistance of the same and the resistance of the voltage-dividing resistor R1 will be reduced. The A/D converter C has a reference voltage input terminal supplied with a voltage obtained by suitably dividing a voltage (V1+V2) imposed on the weight sensor A through voltage-dividing resistors R3, Rr and R4 for achieving tracking with respect to the weight sensor A. The span of the scale can be adjusted by operating the variable resistor Vr.
As is well known, the resistance of the variable resistor Vr is liable to vary due to vibrations, temperature changes, and time. Even with the circuit arrangement of FIG. 1 in which any variation in the resistance of the variable resistor Vr is reduced, the span of the scale is subjected to changes due to a variation in the resistance of the variable resistor Vr. To place such a span change in an allowable range, the range of adjustment achieved by the variable resistor Vr should be reduced to make negligible any influence by variations in the resistance of the variable resistor Vr. However, since the rated output voltage from the weight sensor A suffers a considerably wide variation, the scale span adjustment is sometimes impossible to make if the variable range of the variable resistor Vr is reduced.
It has been conventional practice to change the reference voltage in the A/D converter C, dependent on the output voltage from the weight sensor A, to put any scale span change in an allowable range. Such a change in the reference voltage however requires the selection, soldering attachment, and replacement of the voltage-dividing resistors R3, Rr and R4, and hence the efficiency of connecting such resistors has been low and the cost of manufacture has been high.
It is well known in the art that the scale span is affected by the gravitational acceleration of the earth. Therefore, where the spans of electronic scales, to be used in geometrical areas subjected to different gravitational accelerations, are to be adjusted in a manufacturer's factory, it is necessary for the manufacturer to have many weights for the areas (hereinafter referred to as "area weights"). That is, the manufacturer must take into account the gravitational accelerations of each area, for respective weighing capacities. In the span adjustment, the area of use should be confirmed for each scale in selecting the area weight for that scale.
Accordingly, it has been quite tedious and time-consuming for the manufacturer to make the span adjustment for each area. When areas of use are changed due to a variation in the market demand, the span has to be adjusted again. Therefore, such a change in areas of use has been a tremendous burden on the part of the manufacturer. The cost of keeping the various area weights in inventory has also been a great burden on the manufacturer.